The present invention relates to a torque sensor having a stator that detects changes in magnetic flux passing through a magnetostrictive material fixed on a shaft that is rotatable relative to the stator. The present invention also relates to a member for restricting the rotation of the stator.
Magnetostrictive torque sensors typically include a detecting coil, a shaft and magnetostrictive material provided on the peripheral surface of the shaft. Application of torque on the shaft strains the magnetostrictive material and causes changes in the magnetic permeability of the sensor. The changes in the permeability alter flux and thus induce electromotive force in the detecting coil. The applied torque is detected based on the induced electromotive force. Methods for detecting torque applied on the shaft in such sensors are proposed, for example, in Japanese Unexamined Patent Publication No. 5-118938 and Japanese Unexamined Patent Publication No. 59-77326.
In a typical magnetostrictive torque sensor, a shaft is rotatably supported in a housing. A stator having a detecting coil is fixed to the inner wall of the housing. Also, magnetostrictive material is fixed to the shaft. A predetermined gap exists between the stator and the material. However, eccentric rotation of the shaft relative to the housing varies the distance between the stator and the material and thus degrades the performance of the sensor.
A sensor has been proposed in which a stator is supported on a shaft to overcome this drawback. As shown in FIG. 13, a shaft 51 is supported by bearings 53 in a housing 52 and thus rotates relative to the housing 52. A cylinder 54 made of magnetostrictive material is fixed to the shaft 51. Also, a cylindrical stator 55 is rotatably supported on the shaft 51 by bearings 56. The bearings 56 create a predetermined distance between the inner wall of the stator 55 and the surface of the cylinder 54. The stator 55 includes an exciting coil 57 and a detecting coil 58. An alternating electric current is applied to the exciting coil 57. The current forms a magnetic circuit including magnetic flux through the cylinder 54. Application of torque on the shaft 51 strains the cylinder 54 and causes changes in the flux through the cylinder 54. The flux changes are detected by the detecting coil 58. Terminal wires of the coils 57, 58 are soldered to lead wires 59. The lead wires 59 extend through holes 55a formed in the cylinder 55 and holes 52a formed in the housing 52. This construction maintains the predetermined distance between the stator 55 and the cylinder 54 even if the shaft 51 rotates eccentrically relative to the housing 52.
However, due to friction in the bearings 56, rotation of the shaft 51 applies rotational force to the stator 55. Therefore, when the shaft 51 is rotated, the stator 55 is not always fixed relative to the housing 52. In other words, the stator 55 rotates a little relative to the housing 52 as illustrated in FIG. 14. The rotation of the stator applies tension to the lead wires 59 and the terminal wires. The tension can crack the solder connecting the lead wires 59 with the terminal wires. Thus, the terminal wires are likely to be damaged or broken.
Accordingly, it is an objective of the present invention to provide an improved torque sensor in which a stator is rotatable relative to a rotary shaft. Specifically, it is an objective of the present invention to provide a torque sensor and a stator rotation restrictor that prevent terminal wires of a stator from receiving tension and that are easy to assemble.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque sensor for sensing torque applied to a shaft that is rotatably supported in a housing is provided. The sensor includes a magnetostrictive member fixed to the shaft and an exciting coil for generating flux running through the magnetostrictive member. The magnetostrictive member is strained by the torque applied to the shaft. A generated flux varies in accordance with the strain of the magnetostrictive member. The sensor further includes a detecting coil for detecting the flux variation, a stator for incorporating the exciting coil and the detecting coil such that the coils are located about the shaft, a support for supporting the stator in the housing and a rotation restrictor for preventing the stator from rotating relative to the housing.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.